Linear motors are used in a variety of electrical devices. For example, photolithography systems and other semiconductor processing equipment utilize linear motors to precisely position a wafer stage holding a wafer. Alternately, linear motors are used in other devices, including elevators, electric razors, machine tools, metal cutting machines, inspection machines, and disk drives.
A typical shaft-type linear motor includes a magnet array and a coil array. The magnet array includes a plurality of magnets positioned side-by-side. The magnets generate a magnetic field. Each magnet is cylindrical shaped and includes a longitudinal axis and a pair of opposed sides. Typically, each magnet is axially polarized relative to the longitudinal axis. Stated another way, the opposite poles are located on the opposed sides of each of the magnets.
The coil array includes a plurality of coils that are individually supplied with an electrical current. The electric current supplied to the coils generates an electromagnetic field. The electromagnet field interacts with the magnetic field of the magnet array. This causes the coil assembly to move relative to the magnet array. When the coil array is secured to the wafer stage, the wafer stage moves in concert with the coil array.
One of the factors that influences the performance of a shaft-type linear motor is the performance of the magnet array. The performance of the magnet array can be characterized by the ratio of the flux density of the magnetic field of the magnet array to the mass of the magnet array. The ratio between the flux density and the mass shall be referred to herein as the ("flux/mass ratio"). A high flux/mass ratio indicates an efficient magnet array.
In light of the above, an object of the present invention is to provide a magnet array for a linear motor having an improved flux/mass ratio. It is another object of the present invention to provide a method for increasing flux density without significantly increasing the mass of the magnet array. Another object of the present invention is to provide a method for decreasing the mass without significantly decreasing the flux density. Yet another object of the present invention is to provide a linear motor that dissipates less power for a given force.